Downhole drilling is required in many fields to evaluate subsurface materials and to extract minerals and other natural resources. Such fields include mining and the recovery of hydrocarbons in oilfield drilling. Drilling is also known for use in excavation activities such as for utility installation.
Recovering hydrocarbons from subterranean zones relies on the process of drilling wellbores. Wellbores are made using surface-located drilling equipment which drives a drill string that eventually extends from the surface equipment to the formation or subterranean zone of interest. The drill string can extend thousands of feet or meters below the surface. The terminal end of the drill string includes a drill bit for drilling (or extending) the wellbore. Drilling fluid usually in the form of a drilling “mud” is typically pumped through the drill string. The drilling fluid cools and lubricates the drill bit and also carries cuttings back to the surface. Drilling fluid may also be used to help control bottom hole pressure to inhibit hydrocarbon influx from the formation into the wellbore and potential blow out at surface.
Bottom hole assembly (“BHA”) is the name given to the equipment at the terminal downhole end of a drill string. In addition to a drill bit, a BHA may comprise elements such as: apparatus for steering the direction of the drilling (e.g., a steerable downhole mud motor or rotary steerable system); sensors for measuring properties of the surrounding geological formations (e.g., sensors for use in well logging); sensors for measuring downhole conditions as drilling progresses; systems for telemetry of data to the surface; stabilizers; and heavy weight drill collars, pulsers and the like. The BHA is typically advanced into the wellbore by a string of metallic tubulars (also called drill pipe).
Telemetry information can be invaluable for efficient drilling operations. For example, telemetry information may be used by a drill rig crew to make decisions about controlling and steering the drill bit to optimize the drilling speed and trajectory based on numerous factors, including legal boundaries, locations of existing wells, formation properties, and hydrocarbon size and location. A crew may make intentional deviations from the planned path as necessary based on information gathered from downhole sensors and transmitted to the surface by telemetry during the drilling process. The ability to obtain real time data allows for relatively more economical and more efficient drilling operations. Various techniques have been used to transmit information from a location in a borehole to the surface. These include transmitting information by generating vibrations in fluid in the borehole (e.g., acoustic telemetry or mud pulse telemetry) and transmitting information by way of electromagnetic signals that propagate at least in part through the earth (electromagnetic or “EM” telemetry). Other telemetry systems use hardwired drill pipe or fibre optic cable to carry data to the surface.
The process of transmitting information from a location in the borehole to the surface and other downhole activities can require a downhole power source. For example, with typical measurement while drilling (“MWD”) equipment required for operational control or data analysis, MWD measurements are taken downhole with an electromechanical device located in the BHA. These MWD tools need electrical energy from a power supply for their operation in the borehole. A power supply generally comprises an electrical storage and generator for generating electrical output. The electrical storage could be a chemical battery such as an aluminum electrolytic capacitor, tantalum capacitor, ceramic and metal film capacitor, or hybrid capacitor magnetic energy storage. The electrical storage could also be a mechanical energy storage device such as a fly wheel, spring system, spring-mass system, thermal capacity system, or hydraulic or pneumatic system. In MWD systems, the MWD equipment can be coupled to an electronics package along the drill string, which in turn can be coupled to a power supply along the drill string that provides power to the downhole electronics. MWD can use either battery power systems or turbine power systems, although unlike turbine systems, batteries can provide power to the MWD system independent of drilling-fluid circulation and are necessary if logging will occur during tripping in or out of the borehole. Thus, the typical main energy source for these purposes is batteries. Lithium batteries (such as lithium-thionyl chloride batteries) are commonly used in MWD systems because of their combination of high energy density, even at high MWD service temperatures, and the provision of a stable voltage source until very near the end of their service life, and they usually do not require complex electronics to condition the power supply.
An electrical storage cell typically comprises a pair of electrodes (anode and cathode) comprising electrochemically active positive and negative materials, respectively, each having a respective current collector. The current collectors are metal contacts or leads that form terminals and provide electrical access to the appropriate layer of the energy storage cell. The electrodes are typically separated from one another by a separator. The electrical storage cell oftentimes appears as a thin flat layer with the separator between the electrodes. To construct the electrical storage cell, an insulating sheet is typically laid down, with a thin layer of an anode material on top, a separator layer is applied, and then the cathode material is layered on top. These sandwich layers are then rolled up into a cylindrical cell to form what is often called a “jelly roll” or “Swiss roll”, with respective current collectors projecting at each end of the jelly roll. The wound storage cell can then be secured with a wrapper or packaged in a hollow cylindrical casing and hermetically sealed with a liquid electrolyte. An example of such a prior art design is illustrated in FIGS. 1a and 1b, where a battery coil 1 is configured for retention in a casing 2, which casing 2 is sealed at opposed ends by caps 3. This jelly roll design is the design most commonly used for cylindrical rechargeable batteries such as nickel-cadmium, nickel-metal hydride, and lithium ion, but can also be used for primary or non-rechargeable batteries. FIGS. 2a to 2c illustrate sectional views of conventional cell designs, namely the high-rate “jelly roll” design (FIG. 2a), the moderate-rate construction (FIG. 2b), and the bobbin design (FIG. 2c).
The casing for the jelly roll storage cell can provide structure and physical protection for the storage cell. The casing is typically an annular cylindrically shaped body and a complementary cap or plate on one or both ends of the casing. Current collectors of the electrodes are connected to the electrical connection means of the caps that cover them. The seal between the casing and the caps can prevent access to the internal environment of the storage cell, for example by air and humidity. It also prevents leakage of the electrolyte from the storage cell. When the jelly roll is sealed within the casing, the current collectors are connected to the casing. The current collectors can be electrically coupled to the casing by use of a feed through or can be directly coupled to the casing.
In use, the wound electrodes are surrounded by the liquid electrolyte. The porous separator isolates the electrodes mechanically to prevent an internal short circuit, while allowing ion flow or diffusion to occur. The electrical potential difference between the anode and cathode allows electron flow, or current, to be provided from the anode when a conductive path or electronic device is connected to the battery. The cell will cease producing electric power when electron flow stops for a variety of reasons. Some of these reasons include mechanical failure such as if ions cannot reach the cathode, when the external current path is interrupted, or if the anode contacts the cathode.
In downhole drilling, a long drill string and rotating drill bit drill a wellbore into the Earth, requiring power downhole. The power supply or battery cell could be disposed along the drill string. Since the wellbores that must be drilled into the Earth in these cases are required to be very large and a great amount of energy is consumed while downhole, the batteries that are used in downhole drilling are large industrial-sized batteries.
In MWD tools in particular, the battery is large and is often positioned directly above the drill bit, placing the battery in one of the toughest environments in drilling. The very harsh subsurface operating environments of MWD systems not only include high temperature and pressure, but also downhole shock and vibration that can be problematic with strong lateral and axial shocks to the system. The batteries must be able to withstand the rigorous mechanical shocks and vibrations of the downhole environment, while providing continuous power to operate the tool. Included in the problems with downhole shock and vibration are problems with torsional shock which can be produced by stick/slip torsional accelerations. These shocks may be significant and the tools can be expected to fail if subjected to repeated stick/slip because of mechanical damage to tool string components, including the battery.
Stick/slip is a violent reaction to built-up torsional energy along the length of the drill string. It can occur as the formation strength increases and more weight on bit (“WOB”) is required to maintain efficient depths of cut. The stick/slip phenomenon is a spontaneous jerking motion that can happen as two objects are sliding over one another. The surfaces alternate between sticking to one another and sliding over each other, with a corresponding change in the force of friction. When an applied force is large enough to overcome the static friction between the surfaces, the reduction of the friction to the kinetic friction can cause a sudden jump in the velocity of the movement. Stick-slip can thus occur at the rock-cutting interface where the cutters meet the rock or can be produced by friction between the hole wall and the drill string itself. When stick/slip takes place at the end of the drill string, an accumulation and release of energy stored as several turns of twist in the string can occur. During the “stick” period, the bit stops drilling while WOB and torque on bit (“TOB”) remain being applied. As the rotary table on the rig floor continues to turn, the resulting torque loading on the drill string can cause the bit to eventually give way or “slip”, causing a significant increase in its rotational speed. In this slip or release phase, the string spins out of control and creates the stick/slip-associated vibrations that can be destructive.
The stick-slip vibration can cause periodic fluctuations in bit rotational speed, ranging from zero to more than five times the rotational speed measured at the surface on the rig floor. When mud motors are used, the stick/slip torsional wave to the surface is reduced, but still imparts vibrations that can damage guidance electronics and cause damage to the battery. Stick/slip has been regarded as the most detrimental vibration axis to the service life of downhole equipment because the torsional movement and axial vibrations of stick/slip can generate or result in mechanical stress to the tool and in particular, the battery cell can be deformed due to the rotation of the structure.
As the drill string rotates in the wellbore, there is concurrent rotation of the battery cell casing. The high shock and vibration resulting from the stick/slip phenomenon can cause the inner storage cell to move independently of the casing, causing an unrolling of the wound jelly roll storage cell. This can result in accidental anode and cathode contact, thus short-circuiting the battery. When this happens, heat and gas can be produced in an accelerated chemical reaction and explosions can occur if the cell temperature rises high enough.
Downhole failure of the battery such as this can be both dangerous and expensive. A short circuited battery can cause leakage of the battery and even an explosion. Furthermore, the cost in time and money of replacing damaged batteries situated deep within a wellbore can be significant because doing so requires removing the entire BHA to retrieve the battery and replace it.